"KNOXVILLE -- The earthen dikes supporting a huge coal ash landfill at a Tennessee power plant were 'on the verge of failure' long before they collapsed and sent tons of toxic muck into a river and lakeside community, an engineering consultant said Thursday.

Engineer Bill Walton of AECOM USA Inc. said his firm's $3 million study for the Tennessee Valley Authority found several factors combined to lead to the Dec. 22 dike breach at TVA's Kingston Fossil Plant. Among them was a thin layer of fly ash 'slime' deep in the pile that moved and put pressure on the dikes.

Walton said the disaster could have happened at any moment -- maybe a year before, maybe the next day.

The consultant said it was not his job to assess blame for the spill that has brought national attention to the risks and limited regulation of storing coal ash, which can contain toxic metals such as arsenic, mercury and selenium.

The report's conclusions, however, suggested better engineering and inspections would have helped."